A role of an external additive of the electrostatic latent image developing toner (hereinafter also simply referred to as “toner”), is improvement in chargeability and fluidity may be mentioned, for example. Generally, the external additive is a fine powder of an inorganic oxide, and silica particles, titania particles, and alumina particles are used. Although silica particles are effective for improving fluidity, there is a problem that the charge amount of the toner is excessively increased especially in a tow temperature and low humidity environment clue to high negative chargeability.
Therefore, means for imparting the effect of suppressing the charge amount in a low-temperature and low-humidity environment is known by using silica particle in combination with titania particles having low electric resistance (hereinafter simply referred to as resistance). However, since titania particles have low resistance so that there is a problem that the charge transfer of the carrier particles is promoted and the charge amount of the toner is lowered when transferred to carrier particles during high coverage printing.
Therefore, it is known to increase the amount of the surface modifying agent of the titania particles in order to have the same degree of resistance as the carrier particles. However, in order to make the titania particles have the same degree of resistance as the carrier particles, the surface modification amount becomes excessive. When the excessive surface modifying agent is liberated, the modifying agents aggregate with each other, the fluidity is deteriorated, and there is a problem that the charge amount of the toner is lowered.
As an external additive, it is also known to use alumina particles having higher resistance than titania particles and lower resistance than silica particles. For alumina particles, for example, it is known to use a hydrophobilized material (see Patent documents 1 and 2: JP-A 2009-265471 and JP-A 2009-192722). However, with the conventional method using alumina particles, it was impossible to stabilize the fluctuation of the charge amount at the time of high coverage While securing the fluidity of the toner. There is also known a method using a developer containing a toner containing toner particles to which alumina particles are externally added and carrier particles coated with a thermosetting straight silicone resin (see Patent document 3: JP-A 11-7149). However, since the heat-cured straight silicone resin has high hygroscopicity, there is a problem that it is impossible to sufficiently suppress the fluctuation of the charge amount due to the environment.